Field of the Invention
The present invention relates to a printed circuit board and a printed wiring board, where a receiving circuit is electrically connected to a trunk wiring by branch wiring.
Description of the Related Art
Generally, a memory system is configured including a memory controller and multiple memory devices. Examples of commonly known memory devices include Double Data Rate 3 Synchronous Dynamic Random Access Memory (DDR3-SDRAM).
A memory device may be mounted on a mother board, or may be mounted on a module board. In a case where a memory device is mounted on a module board, a memory system is configured by the memory device being connected to the motherboard to which a memory controller has been mounted.
A memory controller transmits an address signal or command signal (address/command signal), and each memory device is controlled by receiving an address/command signal, with data signals being exchanged between the memory controller and multiple memory devices. Particularly high-functionality electronic devices often use DDR3-SDRAM to secure memory space.
The memory device DDR3-SDRAM has built in a function to adjust signal transmission timing. Each memory device is connected to a so-called “fly-by” trunk wiring where high-speed address/command signals are enabled (see JEDEC standard No. 21C PC3-6400/PC3-8500/PC3-10600/PC3-12800/PC3-14900/PC3-17000 DDR3 Unbuffered SO-DIMM Reference Design Specification Revision 2.0). In this JEDEC standard, the trunk wiring uses two inner layers of the printed wiring board. Also, a ball grid array (BGA) type semiconductor package is used for the memory device in the JEDEC standard. The printed wiring board has formed thereupon vias formed on the trunk wiring to make up branched wiring branching from the trunk wiring to the memory device, mounting pads connected to reception terminals of the BGA type semiconductor package, and leading wiring connecting the vias and mounting pads. The length of the branched wiring to the memory device can be reduced by using small-diameter vias, which can be disposed between mounting pads, for part of the branched wiring.
On the other hand, Japanese Patent Laid-Open No. 2009-86841 proposes reduction in waveform ringing by adjusting the length of the branched wiring on a dual inline memory module (DIMM), in a case of a DIMM having a DDR3-SDRAM mounted to a module board.
However, the length of the branched wiring is set to be short in the above Japanese Patent Laid-Open No. 2009-86841 and the JEDEC standard, around 3 to 5 mm, in accordance with the DIMM standard. Accordingly, the length of the leading wiring has been reduced in the related art by situating the vias adjacent to the mounting pads, thus reducing the length of the branched wiring. However, in the case of wiring of which the number of lines is great, and mounting pads are disposed in high density, such as address/command wiring or the like, vias cannot be disposed between the mounting pads, and may be disposed on the outer side of a mounting pad group. In such a case, the branched wiring will be longer. The longer the branched wiring is, the larger the problem of signal attenuation and reflection becomes, and can lead to disturbance in signal waveforms, i.e., ringing in the signals. Particularly problematic in address/command signal waveforms in DDR3-SDRAMs is that there are cases where signal ringing is large to where input voltage conditions for the signals cannot be satisfied.
It has been found desirable to provide a printed circuit board and a printed wiring board capable or reducing ringing in the waveforms of signals received at a first reception circuit.